The invention relates to a thin-film solar cell on the basis of the IB-IIIA-VIA semiconductors, the back electrode of which consists of intermetallic phases of the same metals which are used for manufacturing the polycrystalline absorbing layer of the p-type conductivity. The invention further relates to a method for manufacturing such a solar cell.
It is one of the main aims of the development of photovoltaic modules to substantially reduce the production costs of solar electricity for terrestrial applications. For extraterrestrial applications the reduction in weight of the solar modules is together with the radiation resistance of the solar cells in the focus of the development.
Solar cells and modules on the basis of polycrystalline IB-IIIA-VIA compound semiconductors are suitable candidates in order to reach these aims. In recent years great progress could be reached particularly with compound semiconductors on the basis of copper, indium and gallium as well as selenium or sulfur as photoactive absorber material which are generally called CIS or CIGS.
The cell structure known from U.S. Pat. No. 5,141,564 comprising a substrate made of glass, a back electrode made of molybdenum, a polycrystalline absorber layer having a thickness of 1 to 5 xcexcm of CuInSe2 or CuIn(Se,S)2 of the p-type conductivity, a thin-film cadmium sulfide-window layer and a transparent front electrode of the n-type conductivity, forms the basis of most methods for manufacturing these polycrystalline thin-film solar cells. Instead on molybdenum-covered glass substrates the polycrystalline absorber layers may also be deposited on flexible tape-like substrates like tapes made of molybdenum or metallic tapes covered by molybdenum as described in German Patent DE 42 25 385 C2 and in European Patent Application EP 0,574,716 A1.
All these arrangements comprise a back electrode made of molybdenum. The low adherence of the polycrystalline absorber layers on molybdenum however leads to peeling of the layers on glass substrates and flaking on flexible tapes and hindered the development of flexible CIS-solar cells so long to a great extent. In order to improve the adherence of CIS-absorber layers, intermediate layers made of titanium, tantalum, chromium or titanium nitride between the molybdenum-back electrode and the absorber layer are proposed in WO 95/09441 A1. In EP 0,360,403 A2 a copper-indium-diselenide solar cell having a gallium containing intermediate layer between the CIS-absorber and the substrate made of molybdenum is proposed. In EP 0,798,786 A2 a solar cell having a chalcopyrite-absorber layer is proposed according to which a thin intermediate layer is arranged between the back electrode made of molybdenum and the absorber layer, said intermediate layer comprising zinc.
It is disadvantageous with respect to all these arrangements that an additional component is to be introduced and that, therefore, an additional technological step for the deposition of these layers is necessary.
In German Patent DE 196 34 580 C2 a method is proposed according to which the CIS-absorber layer is directly deposited onto a copper tape. According to this method the CIS-absorber layer is closely or firmly intergrown with the copper tape such that no peeling or flaking takes place here. In Solar Energy Materials and Solar Cells 53 (1998) pp. 285-298 a thin layer of intermetallic copper-indium phases is indicated between the copper tape and the polycrystalline absorber layer. The absorber layer produced according to this method is of the n-type of conductivity. Since the mobility of the minority carriers is smaller by one order of magnitude in n-type conductivity chalcopyrites than in p-type conductivity chalcopyrites, more reduced efficiencies are to be expected with respect to this arrangement if compared to an arrangement comprising a p-type absorber layer and a n-type conductivity collector layer. Moreover, this method is affected in a detrimental way by the reduced mechanical stability of copper such that relatively thick copper tapes are to be used. Since according to this method the copper tape simultaneously forms the copper source for the absorbing layer, very pure copper tapes are to be used such that the costs of the solar cell are unnecessarily increased.
It is an object of the present invention to provide a solar cell comprising a p-type conductivity absorber layer on the basis of IB-IIIA-VIA compound semiconductors which firmly adheres on any cost-effective carrier film. It is a further object of the present invention to provide a solar cell of the above described type which is flexible and which may be produced at a rather low expenditures. It is still another object of the present invention to provide a solar cell of the above described type having a rather low weight. It is another object of the present invention to provide a method for manufacturing such a solar cell.
These and other objects are solved by a thin-film solar cell on the basis of IB-IIIA-VIA compound semiconductors comprising a back electrode between the polycrystalline IB-IIIA-VIA absorber layer of the p-type of conductivity and a carrier film used as a substrate, said back electrode being made of intermetallic phases of the same IB- and IIIA-metals which are deposited for the generation of the absorber layer.
These and other objects are also solved by a method for manufacturing a thin-film solar cell having a back electrode between the absorber layer and the carrier film, said method characterized in that the IB- and IIIA-metals are deposited on the carrier film, the metals on the side opposite to the carrier film are vertically only incompletely converted by reaction with chalcogen into the photovoltaicly active absorber material such that the intermetallic phases of the IB- and IIIA-metals are directly located on the carrier film, said IB- and IIIA-metals serving as back electrode of the solar cell structure, a buffer layer is deposited, and a transparent conductive front electrode is deposited.
Advantageous embodiments of the present invention are described in the respective sub-claims.
It is the basic idea of the present invention that the back electrode of the solar cell is made of intermetallic phases of the same IB- and IIIA-metals which are used for the formation of the polycrystalline absorber layer. These intermetallic phases simultaneously serve for the coupling between the absorber layer and the flexible carrier film. Thus, a separate deposition of a molybdenum-layer as back electrode and the deposition of intermediate layers for the improvement of the adherence of the absorber layer is not necessary.
With respect to the good adherence of the absorber layer cost-effective, very thin, mechanically rigid metal films may be used as substrate. On these substrates flexible solar cells can be produced by continuous roll-to-roll processes. In a first step the IB- and IIIA-metals are deposited in a continuous roll-to-roll process. In a second continuous roll-to-roll process the carrier film covered by the precursor is chalcogenised in a narrow-slit reactor. The process is performed such that the IB-IIIA metallic precursor is only incompletely converted into the ternary polycrystalline p-type absorber layer such that intermetallic phases of the IB- and IIIA-metals are directly located on the carrier film which metals serve as back electrode of the solar cell structure. Because of the short reaction time during the chalcogenising process and the low heat capacity of the carrier film ternary polycrystalline absorber layers may thus be deposited on thin carrier films in a continuous roll-to-roll process in an effective manner. The polycrystalline absorber layer is closely intergrown with the back electrode such that neither peeling nor flaking will occur on the flexible metal films. The deposition of a buffer layer and of a transparent front electrode in a continuous roll-to-roll process completes the process for producing these flexible thin-film solar cells.